| How to realize the harmlessness,stabilization,reduction and recycling of municipal sludge has become one of the important topics in the field of environmental protection research.Pyrolysis of sludge is an environmentally friendly recycling and disposal technology.Pyrolysis can effectively reduce the volume of sludge,kill pathogens,and convert organic matter in sewage sludge into biological energy?bio-oil and syngas?and biochar product.However,the organic carbon content of sludge biochar is relatively low,and the possible harm of heavy metals of sludge to the environment also cannot be ignored.Meanwhile,the utilization of sludge-based biochar in the field of heavy metal contaminated soil remediation remains to be further studied.In this paper,based on the study on the properties of sludge based biochar,the effects of pyrolysis temperature,waste type and the mixture ratio on the properties of biochar derived from co-pyrolysis of sludge and agricultural and forestry waste were studied.Meanwhile,the migration and transformation behavior and fixation mechanism of heavy metals during the process of co-pyrolysis were also discussed.Besides,the application feasibility of co-pyrolysis biochar and sludge-sawdust co-pyrolysis biochar in heavy metal contaminated soil were discussed,and the effects of sludge biochar,the used waste char and co-pyrolysis biochar on the migration and transformation of heavy metals in soil were also analyzed through the pot experiment.The main results were shown as follows:After pyrolysis,most of the ash in the sludge were retained in biochar?retention rate,83.2?91.2%?,while the volatile content was lost mostly in the pyrolysis process?retention rate,46.6?76.9%?.When the pyrolysis temperature were 400?or 800?,the surface area and pore volume of sludge biochar were higher.And higher temperature pyrolysis was more conducive to improve the adsorption performance of sludge biochar to the two dyes used in the experiment.Therefore,the main properties of sludge biochar were closely related to the pyrolysis temperature.Changing the pyrolysis temperature can affect the adsorption performance of biochar by changing the composition of biochar,but the specific surface area of sludge biochar was mainly determined by the decomposition characteristics of sludge.Under the same pyrolysis temperature?300?600°C?,the yields of co-pyrolysis biochar were lower than that of sludge single pyrolysis,and the corresponding organic carbon contents in co-pyrolysis biochar were higher than that of sludge single pyrolysis,yet the pH of co-pyrolysis biochar did not change significantly.The organic carbon contents in biochar derived from co-pyrolysis of sludge and rice husk ranged from 16.28%to 38.76%,and the corresponding organic carbon contents in biochar derived from co-pyrolysis of sludge and sawdust ranged from 16.28%to 50.82%based on the results of elemental analysis.When the pyrolysis temperature was 500°C and the sawdust mixing ratio was 50%,the maximum organic carbon content in the received biochar was 50.8%,which was 2.98 times that of the single pyrolysis of sludge under the same conditions.Meanwhile,when the pyrolysis temperature was600°C,the degree of carbonization of co-pyrolysis biochar was higher.In conclusion,during the pyrolysis process pyrolysis temperature and mixing ratio were the main factors influencing the properties of biochar,yet the influence of pyrolysis temperature on properties of biochar were greater than the mixture ratio of the selected waste.Meanwhile,the fixed rate of Pb in biochar was above 90%,and that of Cd was above 70%after pyrolysis.When the mixing ratio of rice husk and sawdust in raw material was 50%and the pyrolysis temperature was 600°C,the leachable Pb and Cd in biochar reached the minimum.The results of chemical speciation analysis showed that most Pb and Cd in the received biochar existed in the stable residue state.And when the mixture ratio of rice husk and sawdust was 50%and the pyrolysis temperature was 600°C,the stable Pb and Cd contents in the corresponding biochar reached the highest.The risk assessment codes?RAC?of Pb and Cd in the co-pyrolysis biochar were low risk?LR?or no risk?NR?,which was reduced by 1?2 grades compared with the sludge sample before pyrolysis.The results based on the flourier infrared spectroscopy and X-ray atomic fluorescence spectroscopy showed that Pb and Cd in the sludge were fixed possibly mainly by direct precipitation on the surface of silicon-aluminum compounds to form PbAl2O4 and CdAl2O4 during the pyrolysis process.On the whole,Pb and Cd can be well fixed during the co-pyrolysis process of sludge and the two different waste.Moreover,the co-pyrolysis of sludge and rice husk had better fixation effect on Pb and Cd in biochar than that of sludge and sawdust.Higher mixture ratio?over 50%?was more conducive to the improvement of heavy metal fixation.Based on the mixed culture experiment of the soil and sludge-rice husk co-pyrolysis biochar or sludge-sawdust co-pyrolysis biochar,it was found that the pH value and organic carbon contents of soil both showed an increasing trend after the addition of the two co-pyrolysis biochar.When the soil incubation time was 30 days and the mixture ratio of biochar was 5%,the leachable Pb and Cd in soil were reduced by 75.1%and 11.1%after the addition of sludge-sawdust co-pyrolysis biochar,and the leachable Pb and Cd in soil were reduced by84.5%and 27.7%by sludge-rice husk co-pyrolysis biochar,respectively.The results of chemical speciation analysis showed that the contents of unstable Pb and Cd in the soil decreased after the application of the two biochar.When the time of soil incubation was 30 days,Pb and Cd in soil gradually became stable,and the level of soil ecological risk was not improved after the application of biochar.Moreover,there was no significant difference in the effects of the two biochar types on chemical speciation of Pb in the soil,yet Cd in the soil was more stable under the co-pyrolysis biochar treatment of sludge and rice husk under the same conditions.In conclusion,the addition of sludge-rice husk co-pyrolysis biochar and sludge-sawdust co-pyrolysis biochar in soil could reduce the leaching toxicity of Pb and Cd in soil to some extent,and the sludge-rice husk co-pyrolysis biochar had better stabilization effect on heavy metals in soil than that of sludge-sawdust under the same conditions.The effects of sludge biochar,rice husk biochar,sludge-rice husk biochar on the soil properties and the availability of lead and cadmium in soil were studied by pot experiments.The results showed that the sludge-rice husk co-pyrolysis biochar had less effect on soil pH value,and the effect on soil organic carbon content of soil was between sludge biochar and rice husk biochar.The available contents of Pb and Cd in the soil were both significantly reduced by adding the selected three biochar?P<0.05?.After the addition of the used three biochar,the fixation rates of Pb in soil were 20.3%,15.0%and 15.5%,respectively,and the fixation rates of Cd were 27.5%,16.3%and 26.0%,respectively.However,after soil application of the three biochar,the order of Pb and Cd uptake in maize seedlings under the different treatments was sludge-rice husk co-pyrolysis biochar<sludge biochar<rice husk biochar.It can be seen that sludge-rice husk co-pyrolysis biochar can better inhibit the absorption of Pb and Cd in plants while fixing heavy metals in soil.Based in the Fourier infrared spectroscopy and Raman spectroscopy,it was found that the higher degree of aromatization in sludge-rice husk co-pyrolysis biochar was the main reason for its inhibition of heavy metal absorption by the maize.On the whole,the Pb and Cd stabilization effect of biochar prepared from co-pyrolysis of sludge and rice husk was better than that of sludge biochar and rice husk biochar.The results of this study showed that the co-pyrolysis of sludge and agricultural and forestry waste is an effective method to realize the utilization of sludge resources.Heavy metals in raw materials can be effectively fixed during the pyrolysis process,and the co-pyrolysis biochar has a broad application prospect in the field of soil application. |
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